Nikon D6 Reference Manual page 494

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Voice Memo File Names
Voice memo file names have the form "DSC_nnnn.WAV". The voice memo
has the same file number ("nnnn") as the picture with which it is
associated. For example, the voice memo for the image "DSC_0002.JPG"
would have the file name "DSC_0002.WAV." Voice memo file names can
be viewed on a computer.
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Voice memos for photos recorded with [Adobe RGB] selected for [Color
space] in the photo shooting menu have names of the form
"_DSCnnnn.WAV".
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Voice memos for photos recorded with a prefix other than [DSC]
selected for [File naming] in the photo shooting menu will be recorded
with the selected prefix in place of [DSC].
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Recording Voice Memos